Mixtures of thermoplastic fluoropolymers

ABSTRACT

A low-molecular-weight copolymer of tetrafluoroethylene with units of perfluoro alkyl vinyl ethers having a melt index of ≧30 suitable as a mixing component with a higher-molecular-weight copolymer of the same monomers for producing moldings in injection molding or by extrusion.

This is a continuation of application Ser. No. 09/058,537 filed Apr. 10,1998, now pending, which claims priority from German Patent ApplicationNo. 198 05 832.2, filed Feb. 13, 1998.

DESCRIPTION

1. Field of the Invention

This invention relates to thermoplastic polymers havingtetrafluoroethylene units and perfluoro alkyl vinyl ether units,mixtures of such polymers that contain low molecular weight and highmolecular weight components, and to processes and articles that employsuch polymers.

2. Background

Copolymers of tetrafluoroethylene (TFEs below) and perfluoro alkyl vinylethers having from 1 to 4 carbon atoms in the alkyl moiety (PAVEsbelow), in particular perfluoro n-propyl vinyl ether (PPVEs below) havebeen known for a long time. Such copolymers are commercially availableunder the designation “PFA”. At a PAVE copolymer content of about 2% byweight and greater, these partially crystalline copolymers haveexcellent technical performance, for example exceptional chemicalstability, combined with high service temperatures. They can beprocessed from the melt as thermoplastics, for example by compressionmolding, extruding or injection molding. Preferred applications are,inter alia, extruded pipes, tubes and cable sheathing. Processing fromthe melt takes place at temperatures of from 350 up to 450° C. Underthese conditions, both thermal and mechanical degradation occur.

The thermal degradation takes place predominantly via the thermallyunstable end groups formed in the polymerization, i.e. from the end ofthe chain. The mechanism of this degradation is described in more detailin “Modern Fluoropolymers”, John Wiley & Sons, 1997, K. Hintzer and G.Löhr, Melt Processable Tetrafluoroethylene-Perfluoropropylvinyl EtherCopolymers (PFA), page 223. The degradation can be substantiallysuppressed by converting the thermally unstable end groups into stableCF₃ end groups by postfluorination, as described, for example in U.S.Pat. No. 4,743,658 and DE-C-19 01 872.

Corrosive gases arise during the thermal degradation, and theseconsiderably impair the quality of the final product by metalcontamination or bubble formation, and can corrode tooling andprocessing machinery, The effect naturally increases with fallingmolecular weight (lower melt viscosity).

The mechanical degradation during processing takes place through chainbreakage, recognizable by the increase of the melt flow index (MFI). Itincreases as extrusion speed (shear rate) rises. The resultant loweringof molecular weight considerably worsens the mechanical properties, inparticular the flexural fatigue strength and other long-term properties,such as long-term failure (stress crack resistance). Keeping themechanical degradation within acceptable limits places correspondinglimitations on processing conditions. This applies in particular to theextrusion speed for pipes, tubes and cable sheathing. At higherextrusion speeds, melt fracture (shark skin) also occurs, as with allthermoplastics. Although it is possible to implement higher extrusionspeeds without melt fracture by lowering the molecular weight (higherMFI values), such products do not have adequate mechanical properties.For this reason, PFA products with an MFI value >15 are currently not onthe market.

It is known from WO-A-97/07147 that a marked rise in the extrusion rateis possible, while avoiding melt fracture and with retention of themechanical properties, with partially crystalline copolymers whichconsist essentially of TFE and at least 3% by weight of perfluoro ethylvinyl ether and which have a melt viscosity of not more than 25×10³ Pasat 372° C., with the proviso that the melt viscosity may exceed thisvalue if the content of the ether mentioned exceeds 10% by weight. Theperfluoro ethyl vinyl ether is, however, difficult to obtain, andtherefore all of the marketed products contain PPVE, which is easilyobtainable industrially and is also preferred for the present invention.

DISCLOSURE OF THE INVENTION

A PFA has now been found which has good melt processability and whichcontains at least one high-molecular-weight PFA with an MFI≦15,preferably from 0.01 to 15, and at least one low-molecular-weight PFAwith MFI≧30. The mixtures of the invention are particularly useful inapplications where chemical resistance and high temperature resistanceare important.

The invention therefore relates to mixtures of thermoplasticfluoropolymers essentially comprising units of TFE and subordinateamounts of units of one or more PAVEs having from 1 to 4 carbon atoms inthe alkyl moiety and a total concentration of from 0.5 to 10 mol %, themixture comprising A) at least one low molecular weight component withan MFI_(A)≧30 and B) at least one high molecular weight component withan MFI_(B)≦15. These components are selected in such a way that theratio of the MFI_(A) of component A) to the MFI_(B) of component B) isin the range from 80 to 2500, preferably in the range of from 240 to750.

“Essentially comprising units of TFE and of a PAVE” means that smallamounts, up to about 5 mol %, of other fluoromonomers not containinghydrogen, such as hexafluoropropene or chlorotrifluoroethylene, are notto be excluded. The composition of the copolymer of the two componentsmay differ within the limits mentioned above.

The mixing ratio of high- and low-molecular-weight components may varywithin wide limits and can be determined for the desired application bymeans of simple preliminary experiments. The ratio is generally from10:90 to 90:10, preferably in the range from 25:75 to 75:25 parts byweight and in particular from 60:40 to 40:60 parts by weight.

The invention also relates to a novel low-molecular-weight PFA with anMFI≧30, preferably ≧120 with preference from 120 to 1000, in particularfrom 120 to 700, especially from 200 to 600.

Another aspect of the invention relates to mixtures of the novellow-molecular-weight PFA(s) mentioned with the high-molecular-weightPFA(s) mentioned above, the MFI ratio mentioned above correspondingapproximately to a molecular weight ratio of the high-molecular-weightto the low-molecular-weight component(s) ≧3.5, preferably from 3.5 to10, in particular from 3.5 to 7.

The MFI gives the amount of a melt in grams per 10 min which is extrudedfrom a holding cylinder through a die by the action of a piston loadedwith weights. The dimensions of the die, the piston, the holdingcylinder and the weights are standardized (DIN 53735, ASTM D-1238). Allof the MFIs mentioned here have been measured with a die of diameter 2.1mm and length 8 mm using a superimposed weight of 5 kg and a temperatureof 372° C. The values 0.01 and 1000 are practically the limiting valuesof this measurement method.

For very high MFI values, therefore, it is expedient to reduce thesuperimposed weight to values down to 0.5 kg, and for very small MFIvalues to increase it to values up to 20 kg. The MFI values determinedin this way are recalculated for a superimposed weight of 5 kg.

The present invention further provides a process for making a shapedarticle from the mixtures of the invention. This process involvesproviding the mixture, extruding, compression molding, or injectionmolding the mixture, and preferably, cooling the mixture to provide aself-supporting shaped article.

Still further the present invention provides shaped articles comprisingthe mixture. Examples of such articles include molded or extruded goodssuch as films, pellets, wire and cable insulation, tubes and pipes,containers, vessel liners, and the like.

DETAILED DESCRIPTION

The novel mixtures may be prepared in a conventional manner, i.e. forexample by mixing the pulverulent products, mixing dispersions of thecomponents, or by conducting the polymerization in an appropriate manner(“step polymerization”) with controlled use of initiator and chaintransfer agent, such as short-chain alkanes and haloalkanes, and alsohydrogen. An advantageous procedure here is as follows: at the start ofthe polymerization, for a low desired MFI, relatively little initiatorand relatively little chain transfer agent are metered in. Thesepolymerization conditions are changed at the desired juncture in thepolymerization, depending on the type of composition by weight to beachieved, for example after 50% of the TFE addition, by metering infurther initiator and chain transfer agent, so that the polymer producedas the polymerization continues has the desired high MFI. The desiredhigh MFI may also be created by increasing the temperature during thepolymerization. The advantage of this preparation process is that a“perfect” mixture of the two components is created in situ.

Preference is given to mixing dispersions of the components and workingup the mixture in a manner known per se (U.S. Pat. No. 4,262,101) oradvantageously by mechanical precipitation using a homogenizer, followedby agglomeration by petroleum fractions. After subsequent drying, theproduct is subjected to melt granulation.

Because the two components have very different MFI values, homogeneousmixtures of powders or of melt granules down to the micro range can beproduced only with equipment which is relatively highly elaborate.However, homogeneous mixtures are essential for achieving excellentperformance.

Compared with a PFA having comparable MFI, the novel mixtures aredistinguished by considerably increased extrusion speed without meltfracture. However, as shown by MFI determination before and afterprocessing, this is not at the cost of significantly increaseddegradation.

The novel mixtures have a noticeably increased zero-shear viscosity anda lower complex viscosity at higher shear rates, compared with acommercially available polymer component with identical MFI.

The PFA with MFI≧30 differs from the hitherto conventional grades of PFAin its low molecular weight. It therefore has a relatively large numberof labile end groups, which limit the thermal stability of the material.For relatively stringent requirements therefore it is expedient toconvert the unstable end groups to stable end groups in a manner knownper se by reaction with elemental fluorine (GB A 1 210 794, EP-A-0 150953 and U.S. Pat. No. 4,743,658). It is expedient here to dilute thefluorine with an inert gas and to use this mixture to treat the drypolymer or polymer mixture. The toxic fluorine is then removed byflushing with inert gas. This same process may be used to postfluorinate the mixtures of the invention.

The success of the postfluorination is checked by IR-spectroscopicdetermination of the residual carboxyl and/or carbonyl fluoride endgroups, as described in U.S. Pat. No. 4,743,658. However, completefluorination of the end groups is not necessary. Reduction of thethermally unstable end groups (COOH+COF) to from 10 to 15 end groups/10⁶carbon atoms is sufficient to achieve the desired improvements inproperties. This significantly shortens the reaction time and thereforemakes the postfluorination more cost-effective.

The novel PFA mixture postfluorinated in this way shows nodiscoloration, even at 450° C. It therefore permits higher processingtemperatures and thus a rise in the throughputs in the extrusion oftubes and of sheathing for wires and cables, and also in injectionmolding. A further advantage of the increased high-temperatureresistance is that when production failures occur, the novel PFA mixtureremains for a longer residence time at high temperatures withoutdegradation and thus there is no discoloration or bubble formation atelevated temperature and no corrosion of the processing machinery or ofthe substrates which come into contact with the polymer mixture.

The preferred process for preparing the novel mixtures consists inblending the two components as dispersions, agglomerating these, dryingand melt granulation followed by water-treatment (DE-A-195 47 909) ofthe granules obtained from the melt and, if desired, postfluorination ofthe same.

The novel mixtures are advantageously suitable for producing thin-walledarticles by extrusion or extrusion blow molding and injection molding.The higher processing speeds which are possible here do not have to beobtained at the cost of impairment of properties; on the contrary, theproducts obtained surprisingly have increased stiffness (increasedmodulus of elasticity) and yield stress, i.e. the novel mixtures canresist higher mechanical stresses in particular applications, since anincreased yield stress means an enlargement of the elastic range ofthese materials. This makes it possible to create moldings with longerservice lives, and this in turn permits the use of tubes with thinnerwalls.

The polymerization may be carried out by known processes of aqueousfree-radical emulsion polymerization (U.S. Pat. No. 3,635,926, U.S. Pat.No. 4,262,101), or in a non-aqueous phase (U.S. Pat. No. 3,642,742).

The perfluoro propyl vinyl ether content is determined by IRspectroscopy (U.S. Pat. No. 4,029,868).

EP-B-362 868 has already disclosed mixtures of fluoropolymers, includinginvestigation of high-molecular-weight and low-molecular-weight PFAgrades. The low-molecular-weight component here is defined by a meltviscosity at 380° C. of from 5000 to 280,000 Poise, corresponding to anMFI at 372° C. of from 80 to 1.6. It is expressly mentioned here that amelt viscosity of less than 5000 Poise (MFI>80) leads to poor mechanicalproperties of the mixture. In the mixture described as example inEP-B-362 868, column 4, the mean molecular weights of the PFA gradesused differ only slightly, to be specific approximately only by a factorof 1.5, corresponding to the melt viscosities of 8.1×10⁴ and 1.9×10⁴Poise, respectively. Such materials are particularly suitable forthick-walled extruded articles, such as pipes.

The invention is described in more detail in the following examples.Percentage and ratio data are based on weight unless otherwise stated.Degradation behavior is assessed using the ratio of MFI after and beforeprocessing.

EXAMPLE 1

25 1 of demineralized water and 122 g of ammonium perfluorooctanoate inthe form of a 30% strength solution are placed in a polymerizationreactor having a total volume of 40 1 and provided with an impellerstirrer. After the reactor has been sealed, atmospheric oxygen isremoved by alternate evacuation and flushing with nitrogen, and thevessel is heated to 60° C. 46 g of methylene chloride and 0.180 kg ofPPVE are pumped in. The stirrer is set at 240 rpm. TFE is thenintroduced until the total pressure has reached 13.0 bar. Thepolymerization is initiated by pumping in 6.6 g of ammonium persulfate(APS below), dissolved in 100 ml of demineralized water. As soon as thepressure begins to fall, further TFE and PPVE are supplemented via thegas phase in accordance with the target ratio of PPVE (kg)/TFE (kg) of0.042, in such a way that the total pressure of 13.0 bar is maintained.The heat liberated is dissipated by cooling the vessel wall, and in thisway the temperature of 60° C. is held constant. After a total of 7.2 kgof TFE has been fed into the reactor, the monomer feed is interrupted,the pressure in the reactor is released and the reactor is flushedseveral times with N₂.

The resultant amount of 31.5 kg of polymer dispersion with a solidscontent of 22.8% is discharged from the bottom of the reactor. After thedispersion has been transferred into a 180 1 stirring vessel, its volumeis increased to 100 1 with demineralized water and it is mixed with 200ml of concentrated hydrochloric acid and stirred until the solid hasseparated from the aqueous phase. The flocculant powder precipitatedafter stirring is granulated with 6.9 1 of a petroleum fraction, thepetroleum fraction is driven off using steam, and the granules are thenwashed six times by vigorous and thorough stirring with 100 1 ofdemineralized water on each occasion. The moist powder is dried for 12hours at 260° C. in a drying cabinet under nitrogen. This gives 7.1 kgof a low molecular weight bipolymer according to the invention which hasa PPVE content of 3.9% and an MFI of 40.

EXAMPLE 2

A PFA mixture according to the invention having an MFI of 2.3 isprepared from a 50/50 mixture composed of a dispersion of the materialfrom Example 1 and a dispersion of a PFA having an MFI of 0.5. The ratioof MFI_(A) to MFI_(B) is 80.

In preparing the PFA having an MFI of 0.5, the procedure is as inExample 1, but 6.7 g of methylene chloride and 1.8 g of APS are pumpedin, giving a bipolymer which has 3.9% of PPVE and an MFI of 0.5.

The dispersion mixture is worked up as in Example 1. This gives abipolymer which has a PPVE content of 3.9% and an MFI of 2.3. After meltgranulation, the MFI rises to 2.4.

EXAMPLE 3

The PFA mixture of Example 2 is compared with a commercially availablePFA having an MFI of 2 in the extrusion of a tube having an externaldiameter of 28.3 mm and an internal diameter of 27.7 mm.

Extruder data: Diameter:   50 mm Length:  1200 mm (length: diameterratio = 24) Compression ratio: 2.5:1 Die: Outer annulus diameter:   60mm Inner annulus diameter:   55 mm Parallel portion:   25 mmCalibration: Diameter:  28.4 mm Extrusion speed: Standard setting:  2.3m/min at 22 rpm Throughput:    8 kg/h Tube weight:   60 g/m Temperaturecontrol: Barrel 1 (Feed):   340° C. Barrel 2:   355° C. Barrel 3:   370°C. Barrel 4:   375° C. Flange:   310° C. Head:   376° C. Die:   388° C.

The results are shown in the following table, the meanings ofabbreviations being

PFA2: Commercially available product with an MFI of 2

TS: Ultimate tensile strength N/mm²

EB: Elongation at break %

Y: Yield stress N/mm² (in each case in accordance with DIN 53455/ASTM D1708, measured in longitudinal and transverse direction on tes specimensstamped out from the tube).

Mechanical properties Throughput MFI MFI Longitudinal TransverseMaterial (kg/h) before after Rise TS EB Y TS EB Y PFA2 8 2 2.7 1.35 26300 12 32 350 12 PFA2 13.5*) 2 2.7 1.45 28 320 12 30 340 12 Example 2 82.4 2.8 1.17 28 340 13 29 360 13 Example 2 20*) 2.4 3.3 1.38 27 320 1330 390 13 *)highest throughput possible without melt fracture

Therefore whereas the commercially available product PFA2 permits only amaximum throughput of 13.5 kg/h, the mixture of Example 2 allows athroughput of 20 kg/h, without adverse effects on the quality of thetube. The MFI change shows that the commercially available product, evenat a low throughput of 8 kg/h, is degraded to about the same extent asthe novel material from Example 2 at a throughput of 20 kg/h.

The yield stress of the novel material is increased. This means that thefinal article has a higher dimensional stability and/or stiffness.

The tubes extruded with the mixture of Example 2 prepared according tothe invention also show, compared with the commercially available PFA2material, increased cold bursting strength.

Using the mixture of Example 2 prepared according to the invention andthe commercially available PFA2 material, and under the same conditions,pipes of 1 mm wall thickness and 10 mm diameter were extruded and theircold bursting strength determined.

The test took place on a bursting strength test apparatus (in-houseconstruction), in which a firmly secured plastic pipe was filled withwater and placed under pressure using a pneumatic pump. The pressuretest is regarded as having been passed if the pipe survives withoutdamage after pressure has been maintained for 6 min at a test pressuredependent on the dimensions of the pipe. After this test has beencarried out, the test pressure is raised by 2 bar/min until the pipebursts, in order to determine the residual bursting strength.

The specified test pressure for pipes of this size is 22 bar.

Residual bursting Materials Pressure test strength [bar] Example 2Passed 27 PFA2 Some passes 24 Some buckling

EXAMPLE 4

The PFA mixture of Example 2 is processed to give a pressed sheet, andlong-term failure is determined on specimens of this pressed sheet. ThePFA2 defined in Example 3 served as comparison. Whereas the mean valueof the times to failure for PFA2 is 194 h, after 793 h only two of threespecimens of the mixture of Example 2 had failed.

The tests were long-term tensile creep tests based on the specificationof the Deutscher Verband für Schweisstechnik [German Association forWelding Technology], DVS 2203, Part 4, on notched specimens. Thespecimens were compression-molded plates of 5 mm thickness. The forceapplied was 4 N/mm². The medium used is demineralized water containing2% of non-ionic surfactant (ARKOPAL® N 100). The tests are carried outat a temperature of 80° C. In each case, the measurements are carriedout on three identical test specimens. This test method, and thereforealso the results, permit correlation with DIN 8075 measurements of theeffects of long-term internal hydrostatic pressure on pipes.

Material Time to fracture (mean calculated from three values) PFA 2  194h Example 2 >793 h

EXAMPLE 5

The procedure of Example 1 is followed, but 200 g of methylene chlorideand 20 g of APS are pumped in, resulting in a low molecular weightbipolymer according to the invention having a PPVE content of 4% and anMFI of 500.

EXAMPLE 6

A PFA mixture according to the invention having an MFI of 9.8 isprepared as agglomerate from a 50/50 mixture composed of a dispersion ofthe material from Example 5 and a dispersion of a PFA with an MFI of1.6. The ratio of MFI_(A) to MFI_(B) is 312.5.

In preparing the PFA with the MFI of 1.6, the procedure is as in Example1, but 19 g of methylene chloride and 2 g of APS are pumped in, giving abipolymer which has 4.2% of PPVE and an MFI of 1.6.

The dispersion mixture is worked up as in Example 1. This gives abipolymer which has a PPVE content of 4.1 mol % and an MFI of 9.8.

EXAMPLE 7

The PFA mixture of Example 6 (MFI 9.8) is compared with commerciallyavailable products in pellet form having an MFI of 10 (for examplePFA10) in the injection molding of specimens. For this the materials arefirstly converted into melt pellets, the MFI changing as shown in thetable.

Dumbbell specimens: Heating: Temperature in Zone 1: 390° C. Temperaturein Zone 2: 390° C. Temperature in Zone 3: 420° C. Temperature in Zone 4:350° C. Injection pressure: 600 bar (6-10⁷ Pa) Injection rate: 4 mm/sMold temperature: 210° C.

Results: Modulus of Yield MFI MFI in elasticity stress EB TS Materialpellets specimen [N/mm²] [N/mm²] [%] [N/mm²] Degradation Example 6 11.513.2 642 15.5 468 23.5 1.15 PFA10 10 11.7 593 14.8 450 27.0 1.2

Modulus of elasticity and yield stress are measured on dumbbellspecimens (DIN 53455, Test specimen No. 3) by the DIN 53457 measurementmethod. The novel material shows lower degradation, higher modulus ofelasticity and higher yield stress, without change in mechanicalproperties, such as TS and EB.

The improved flowability of the novel mixture is also apparent in theinjection molding of spirals. The greater the length of the injectedspiral, the better the flow performance. The degradation occurring inthis procedure can be assessed from the MFI ratio.

The injection conditions are as follows: Heating Program 1 Program 2Temperature in Zone 1 435° C. (390° C.) Temperature in Zone 2 435° C.(390° C.) Temperature in Zone 3: 420° C. (380° C.) Temperature in Zone4: 350° C. (350° C.) Injection pressure: 600 bar 700 bar

Results: Heating Degradation Material program LengthMFI_(spiral)/MFI_(starting material) Example 6 1 26.1 2.5 PFA10 1 22.92.45 Example 6 2 23.1 2.2 PFA10 2 severe delamination

Compared with standard material, the PFA mixture of Example 6 showsmarkedly better flowability with the same degradation and a lowertendency to delaminate when lower temperatures and higher injectionrates are used.

EXAMPLE 8

The PFA mixture of Example 6 is converted into melt pellets which showan MFI of 11. 1.5 kg of this mixture is melted in the melt container ina convection heating cabinet at 370° C. for 5 h, and injection moldedwithin a period of 4 min into a mold, likewise heated to 370° C. andhaving complicated injection geometry. The shape to be encapsulated isthat of a magnetic coupling. After cooling for 30 min with water, themolded specimen has no defects, in particular neither gas inclusions norany discoloration. The MFI of the molding is 11.3. In contrast, astandard PFA with MFI 10 or 15 showed delaminations in the molding,making the component unusable.

EXAMPLE 9

125 kg of PFA mixture from Example 6 are placed in a 300 1 tumblerdryer. During heating to 220° C., atmospheric oxygen and moisture areremoved by alternate evacuation and flushing with nitrogen. The reactoris then filled with an F₂/N₂ mixture containing 10% of F₂. The reactionproceeds for 5 hours, and after each hour the F₂/N₂ mixture is renewed.During cooling from 220° C. to room temperature, unreacted fluorine inremoved by alternate evacuation and flushing with N₂. The resultantproduct has only about 15 remaining COOH end groups, corresponding toabout 10% of the thermally unstable end groups initially present.

The resultant product was injection molded essentially as described inExample 7. It is apparent during this that the postfluorinated PFAmixture of Example 9 can withstand higher thermal stresses.

Dumbbell specimens: DIN 53455, Test specimen No. 3 Heating: Temperaturein Zone 1: x₁ Temperature in Zone 2: x₂ Temperature in Zone 3: 420° C.Temperature in Zone 4: 350° C. Temperature [° C.] Material from Zone x₁Zone x₂ Example 6 Example 9 390 390 colorless colorless 400 400colorless colorless 410 410 yellowish colorless 420 420 brownishcolorless 430 430 brown colorless 440 440 brown colorless 450 450 deepbrown colorless

EXAMPLE 10

The procedure (i.e. the preparation of the polymerization reactor) thepolymerization conditions, and the work-up) of Example 1 is followed.However, for preparing a novel mixture by step polymerization, at thestart of the polymerization 7 g of methylene chloride and 2 g of APS areadded. Following 50% of the amount of TFE to be run in, 35 g ofmethylene chloride and 10 g of APS are metered in. This gives abipolymer having a PPVE content of 3.9% and an MFI of 2.1.

The first part of the polymerization gives a PFA having an MFI of 0.3.The MFI created in the second step is calculated from the MFI of 2.1 ofthe end product via the following equation:${MFI}_{A} = \left( \frac{{MFI}_{End}^{- 0.294} - {x \cdot {MFI}_{B}^{- 0.294}}}{x} \right)$x = proportion  by  weight

The MFI is therefore 75. The ratio of MFI_(A) to MFI_(B) is 250.

The PFA mixture of the invention created in this step polymerization wascompared with a standard material PFA2 in a high-pressure capillaryrheometer, in relation to the shear rate at which melt fracture occurs.

Compared with the commercially available material PFA2, the shear rateat which melt fracture just becomes visible is increased by a factor of2 in the material of Example 10.

Shear rate at start of melt fracture in s⁻¹ PFA2 15 Example 10 30

EXAMPLE 11

The procedure (i.e., the preparation of the polymerization reactor, thepolymerization conditions, and the work-up) of Example 1 is followed.However, for preparing a novel mixture by step polymerization, at thestart of the polymerization 3 g of methylene chloride and 2 g of APS areadded. Following the addition of 30% of the amount of TFE to be run in,100 g of methylene chloride and 10 g of APS are metered in. This gives abipolymer having a PPVE content of 3.9% and an MFI of 2.6, and a swellindex of 1.54. The swell index is defined by the following formula:[DE/DD−1]100, where DE is the diameter of the extrudate and DD thediameter of the die.

The first part of the polymerization gives a PFA having an MFI of 0.1.An MFI of 130 created in the second stage is calculated from the MFI of2.6 of the end product, using the equation given in Example 10. Theratio of MFI_(A) to MFI_(B) is 1300.

The material was processed on a continuous extrusion blow molding plantto give 1 1 volumetric flasks and compared with a commercially availableproduct having MFI=2 and a swell index of 1.1. High swell indices areparticularly advantageous for this processing technology.

The processing conditions are as follows: Melt temperature: 370° C.Extrusion speed: 100 mm/min Tube diameter:  60 mm Maximum blow-up ratio:2.5:1

Using the novel material, in contrast to the commercially availableproduct, it was possible continuously to produce, without scrap, 1 1volumetric flasks with uniform wall thickness and wall thicknessdistribution. Using the commercially available product, this issuccessful only with volumetric flasks having a volume of up to 100 ml.

What is claimed is:
 1. A shaped article comprising a mixture ofthermoplastic PFA fluoropolymers consisting essentially of units oftetrafluoroethylene, from 0.5 to 10 mol % of units of one or moreperfluoro alkyl vinyl ethers having from 1 to 4 carbon atoms in theperfluoroalkyl radical and up to about 5 mol % of other fluoromonomersnot containing hydrogen, the mixture comprising: at least 10% by weightof the mixture and not more than 90% by weight of the mixture of atleast one component A) with a melt flow index (MFI_(A))≧30 g/10 min. andnot more than 90% by weight of the mixture and at least 10% by weight ofthe mixture of at least one component B) with a melt flow index(MFI_(B))≦15 g/10 min., the components being selected in such a way thatthe ratio of MFI_(A) to MFI_(B) is in the range from 80 to
 2500. 2. Ashaped article according to claim 1 comprising a film, a pellet, wireinsulation, cable insulation, a tube, a pipe, a container, and a vesselliner.
 3. A process for making a shaped article comprising i) providinga mixture of thermoplastic PFA fluoropolymers consisting essentially ofunits of tetrafluoroethylene, from 0.5 to 10 mol % of units of one ormore perfluoro alkyl vinyl ethers having from 1 to 4 carbon atoms in theperfluoroalkyl radical and up to about 5 mol % of other fluoromonomersnot containing hydrogen, the mixture comprising: at least 10% by weightof the mixture and not more than 90% by weight of the mixture of atleast one component A) with a melt flow index (MFI_(A))≧30 g/10 min. andnot more than 90% by weight of the mixture and at least 10% by weight ofthe mixture of at least one component B) with a melt flow index(MFI_(B))≦15 g/10 min., the components being selected in such a way thatthe ratio of MFI_(A) to MFI_(B) is in the range from 80 to 2500; and ii)forming a desired shape from the mixture.
 4. A process according toclaim 3 wherein the article is formed by extrusion, compression moldingor injection molding.